# Absolute Dating Techniques

History I · Archaeology · https://tryals.app/learn/history-i/absolute-dating-techniques

## Putting Numbers on the Sequence

Stratigraphy orders events; **absolute dating** hangs calendar numbers on the sequence. Choose the wrong method, and it is like timing a sprint with a sundial.

| Method | Clock | Dates | Range |
| --- | --- | --- | --- |
| **Radiocarbon** | Decay of carbon-14 | Once alive | to ~50,000 yr |
| **Dendrochronology** | Annual tree-ring widths | Preserved timber | to ~12,000 yr |
| **Potassium-argon** | Argon in cooled lava | Volcanic rock | 100k to billions |
| **Thermoluminescence** | Radiation since heating | Pottery, flint | to ~500,000 yr |

Living things constantly exchange carbon with the atmosphere, holding a steady trace of radioactive **carbon-14**. Death stops intake; C-14 then decays with a **half-life of 5,730 years** ($t = 5730 \times \log_2(N_0 / N)$ or $N(t) = N_0 \cdot (1/2)^{t / 5730}$). Half the original C-14 means one half-life; a quarter means two.

## The Fine Print & Calibration

Past ten half-lives almost nothing survives, hence the ~50,000-year ceiling. This is why dinosaurs are radiocarbon's most popular impossible request.

Atmospheric C-14 was never perfectly constant, so raw radiocarbon years drift from calendar years. The fix is **calibration**: tree-ring sequences of known calendar age provide a correction curve back through the millennia.

> **Date the death, not the use:** C-14 dates when the organism _died_. A hearth burning 300-year-old driftwood dates the tree, not the fire (the **old wood problem**). A reused oak beam dates its felling, not the house's later renovation. The lab delivers a number; the archaeologist supplies what it means.

## Practice questions

5 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Radiocarbon dates a piece of wood, and archaeologists warn about the 'old wood problem'. What is being dated versus what is wanted?

A. The date the wood stopped exchanging carbon, its death
B. The date of the surrounding sediment rather than the wood itself
C. The date of contamination by modern carbon during excavation
D. The date the wood was buried, which may postdate the event

**Answer:** A. The date the wood stopped exchanging carbon, its death

**Why:** The method dates the sample's death rather than its use, so a beam from an old tree reused in a later building can be centuries adrift. Short-lived materials — seeds, twigs, a single year's growth — are preferred precisely because their death and their deposition are close together.

Page: https://tryals.app/practice/history-i/absolute-dating-techniques/radiocarbon-dates-a-piece-of-wood-and-archaeologists-warn-about-the

### 2. Radiocarbon cannot date dinosaur fossils. What limits it?

A. Its half-life leaves no measurable C-14 after ~50,000 years
B. Contamination from modern carbon makes all fossil dates unreliable
C. The method works only on material that was once alive, which fossils were not
D. Dinosaur bones are too large for the sample sizes the method requires

**Answer:** A. Its half-life leaves no measurable C-14 after ~50,000 years

**Why:** Two independent reasons. After about ten half-lives nothing measurable remains, and a permineralised fossil is rock, which is why potassium-argon and uranium-lead date the surrounding strata instead.

Page: https://tryals.app/practice/history-i/absolute-dating-techniques/radiocarbon-cannot-date-dinosaur-fossils-what-limits-it

### 3. Match each dating method to its essential limitation.

**Answer:**

- Radiocarbon → Fades out past roughly 50,000 years
- Dendrochronology → Needs preserved wood joined to a master sequence
- Potassium-argon → Only volcanic material; useless for recent millennia
- Thermoluminescence → Resets only when the object was last heated

**Why:** Every clock has a dial it cannot read: range, material, and what event actually zeroes it. Matching limitation to method is what separates a date from a blunder.

Page: https://tryals.app/practice/history-i/absolute-dating-techniques/match-each-dating-method-to-its-essential-limitation

### 4. Willard Libby demonstrated radiocarbon dating, revolutionizing prehistory overnight, in which year? **Set the slider.**

**Answer:** 1949 (within ±7)

**Why:** 1949. Before Libby, prehistoric chronologies were educated guesswork chained to Egypt; after him, every once-living thing carried its own clock.

Page: https://tryals.app/practice/history-i/absolute-dating-techniques/willard-libby-demonstrated-radiocarbon-dating-revolutionizing

### 5. Radiocarbon dates a bone to 3,000 BP ± 40 years; dendrochronology can give an exact calendar year. Why is the tree-ring date stronger evidence even when the two agree?

A. Because ring counting is measured and radiocarbon is modelled
B. Because radiocarbon cannot be used on material older than 2,000 years
C. Because the ± 40 means a 40% chance the radiocarbon date is wrong
D. Because tree rings are unaffected by climate and so never ambiguous

**Answer:** A. Because ring counting is measured and radiocarbon is modelled

**Why:** One counts years; the other converts a measured ratio through a calibration curve. That is exactly why radiocarbon must be calibrated against tree rings and not the reverse.

Page: https://tryals.app/practice/history-i/absolute-dating-techniques/radiocarbon-dates-a-bone-to-3-000-bp-40-years-dendrochronology-can